Study the temporal sequence of events that leads to loosening after implantation of total hip replacements in sheep. Attention is directed primarily to the bone-cement interface. The sheep are walked daily on concrete to promote loosening. The hips are studied clinically, radiologically, histologically, mechanically, stereologically, chemically (for free methylmethacrylate monomer) and bacteriologically. The clinical parameters are related to pre-clinical loosening as defined by the loss of torsional rigidity of the cement fixation of the prosthetic components.